The measurement of respiratory flow in patients having respiratory difficulties is extremely important. While there are numerous present methods of measuring such flow, each of them suffers from serious defects. The most common method employed to measure such flow is a resistance pneumotachograph of which the most popular is the Fleisch unit. The resistance pneumotachograph utilizes the principal of measuring a differential pressure caused by an air flow across a resistance in a tube. Prior to the pneumotachograph such resistance was provided by the usual simple orifice. However, with the usual orifice provided for such measurements, flow at the orifice itself becomes turbulent rather than laminar. With turbulent flow, the resistance increases with the flow and the differential pressure measurement does not produce a linear representation of the actual flow. But when flow is laminar, the resistance remains almost constant over a wide range of flow. In order to maintain the flow through the resistance laminar, the Fleisch-type resistance pneumotachograph comprises essentially many small parallel tubes which not only produce the resistance to the flow but also laminate that flow thereby eliminating the effects of turbulence. By measuring the pressure on each side of the small parallel tubes the amount of flow can be determined. Since the flow is laminar, the resistance to flow is constant over a wide range and the differential pressure measurement then is a linear representation of the actual flow of gas.
While the resistance pneumotachogrpah is highly advantageous from a theoretical point of view, it does have serious drawbacks including substantially large size and weight and the small tubes being subject to plugging by either mucus of moisture from the humid air way. Plugging from moisture can be eliminated by heating the device but the problems of mucus together with size and weight are serious and present a substantial drawback to the use of the pneumotachograph.
Other flow measuring devices have been employed for measuring flow at the mouth during pulmonary function testing. In devices for such short term tests there is frequently a large dead space volume and, in addition, they are frequently unstable when water is present in the system.
Still other attempts to measure respiratory flow have been made including the development of ultrasonic flowmeters, hot wire flowmeters, vortex counters and many others. Each of these has its own disadvantage.